Utility companies in the Unites States burn coal and other fossil fuels in a combustion furnace, to generate steam in connection with the electrical power generation process. Coal is burned at a temperature of several thousand degrees Fahrenheit. The by-product of the combustion process is an ash material which may "clinker" or fuse together into large pieces. The ash pieces often collect and drip from the walls of the combustion furnace. When the ash is present in the furnace, the ash is at approximately the same temperature as the combustion gases in the furnace. The heavier pieces of ash, which fall to the bottom of the furnace, are referred to as "bottom ash".
Typically, a large container, which is sometimes referred to as a "hopper", is located beneath the furnace to collect the falling bottom ash. The hopper is constructed of structural steel and is lined on the inside with several inches of concrete refractory. The hopper is usually filled with a liquid, such as water, which is at a relatively high temperature. The refractory insulates the steel material from the hot water inside the hopper, to prevent burn injuries to personnel who come in contact with the steel material on the outside of the hopper. Furthermore, the water inside the hopper may become acidic or caustic. The refractory provides a corrosion protective layer between the steel material and the water. The refractory also provides an abrasion-resistant liner, to prevent the falling ash from contacting the steel material. The water inside the hopper cools the pieces of falling ash and facilitates break up of large pieces of ash. A constant supply of water is provided to the hopper.
The hopper is generally four-sided, with two vertical walls and two sloped walls. The bottom of the hopper includes a moveable door panel, to allow the ash sediment collected at the bottom of the hopper to be emptied. Hoppers may be constructed in various configurations, including single and multiple section hoppers.
Because of the massive size of the hopper, the extremely high temperatures of the furnace above the hopper and other conditions inside the hopper, such as the constant stream of ash pieces descending through the water in the hopper and the turbidity of the water inside the hopper, it is virtually impossible to visually inspect the interior of the hopper to determine how much ash sediment has been collected or to determine the location of the ash sediment in the hopper.